A wide variety of small, portable electronic devices rely on battery power. A regulator circuit may be used to control charging and discharging of the battery. The regulator circuit may allow an external charger to charge the battery in pulses rather than continuously to reduce heating and allow time for the battery to cool between charging pulses. The rate of pulses and pulse width of charging pulses may be adjusted to control temperature.
The regulator circuit may also be used to regulate discharging of the battery, when the battery supplies power to the portable electronic device. The regulator circuit may have a power transistor that is turned on and off to regulate a power voltage. The regulated power voltage may be compared to a reference voltage and fed back to the regulator circuit to control the power transistor. For example, the power transistor can be turned on when the regulated power voltage is too low, and turned off when the regulated power voltage is at or above the target voltage.
FIG. 1 shows a prior-art battery-charging system. Battery 208 is protected by power transistor 210, which has its gate controlled by charge/discharge regulator 206. Battery 208 can drive load 204, which can be a portable electronic circuit or device. When charger 202 is attached, charger 202 may drive load 204 while charging battery 208 by driving charging current through power transistor 210.
Power transistor 210 can be a p-channel transistor with a parasitic diode 212, such as from a p+ drain to the n-well substrate. Diode 212 can protect battery 208 from inadvertent discharge. The “key-chain problem” occurs when a portable electronic device is carried in a person's pocket and has its charging terminals accidentally shorted by metal keys in the person's pocket. Shorting the two terminals to the left of charge/discharge regulator 206 in FIG. 1 could cause battery 208 to be rapidly discharged, except that diode 212 would block this large discharge current. Thus diode 212 provides some protection against the key-chain problem.
However, diode 212 does not protect against large currents in the other direction. If the wrong kind of charger 202 is attached by mistake, a large current could flow through diode 212 to battery 208 even when power transistor 210 is turned off. This large current could damage battery 208.
FIG. 2 shows a prior-art battery-charging system with two power transistors in series. A second power transistor 216 is added in series with power transistor 210. Charge/discharge regulator 206 controls both power transistors 210, 216. A second parasitic diode 214 is formed with power transistor 216. Second parasitic diode 214 may be a P+ source to N-well, while parasitic diode 212 is a P+ drain to N-well.
Since second parasitic diode 214 is in the opposite direction to parasitic diode 212, current in either direction is blocked when power transistors 210, 216 are turned off by charge/discharge regulator 206. Battery 208 is protected from both rapid discharge due to key-chain shorting, and from rapid charging currents due to charger 202. Charger 202 may not be a smart charger and may tend to provide too large of a charging current.
While useful, having two power transistors 210, 216 in series is undesirable, since the effective ON-resistance is increased, or a larger transistor size is needed. The battery charger is less effective since the larger resistance of the regulator power transistors 210, 216 reduces the power-supply voltage provided to load 204 and increases power loss and heating.
What is desired is a battery regulator system that uses only a single power transistor. A single-power-transistor charge regulator is desired that still provides protection against inadvertent shorting caused by the key-chain problem, and against damage to the battery from large charger currents. A charge regulator that does not have latch-up problems is also desirable.